The invention relates to making parts out of composite material comprising a porous substrate densified by a matrix, and it relates more particularly to making parts that have a central passage.
The invention is particularly, but not exclusively, applicable to making annular brake disks or diverging portions or throats for rocket engine nozzles out of thermostructural composite materials.
Thermostructural composite materials are remarkable for their high mechanical properties and their ability to retain these properties at high temperature. Typical examples of thermostructural composite materials are carbon-carbon (C—C) composites comprising a porous reinforcing substrate of carbon fibers densified by a matrix of carbon, and ceramic matrix composites (CMCs) comprising a porous reinforcing substrate of refractory fibers (e.g. carbon fibers or ceramic fibers) densified by a ceramic matrix (e.g. silicon carbide).
Methods of densification by means of chemical vapor infiltration (CVI) are well known. One or more porous substrates are placed inside an enclosure. A gas containing one or more precursors of the matrix-constituting material is introduced into the enclosure. Temperature and pressure conditions are adjusted so as to enable the gas to diffuse within the pores of the substrates so as to deposit the matrix-constituting material by means of one of the components of the gas decomposing or by means of a reaction between a plurality of components. Various gaseous precursors enabling carbon or ceramic matrices to be obtained are well known.
Various known methods are briefly outlined below in the context of making annular disk brakes out of C—C composite material, it being understood that these methods are applicable to making other annular parts (i.e. parts having a central passage), whether out of C—C composite material or out of some other composite material.
FIG. 1 is a highly diagrammatic view of an enclosure 10 containing a load of annular preforms or substrates 20 of carbon fibers. The load is in the form of a stack of substrates having their central passages in vertical alignment. The stack may be made up of a plurality of superposed sections separated by one or more intermediate support plates 12.
The stacked substrates are separated from one another by means of spacers 30. As shown in FIG. 2, the spacers 30 may be disposed radially, and the number of them may vary. They provide gaps 22 of substantially constant height throughout the entire stack between adjacent substrates, while allowing the inside volume 24 of the stack, as constituted by the aligned central passages of the substrates, to communicate with the outer volume 26 situated outside the stack and inside the enclosure 10.
In the example of FIG. 1, the enclosure contains a single stack of substrates. In a variant, a plurality of stacks of substrates may be disposed side by side in the same enclosure.
The enclosure 10 is heated by means of a susceptor 14, e.g. made of graphite, which serves to define the enclosure 10 and which is inductively coupled with an induction coil 16 situated outside a casing 17 surrounding the susceptor. Other methods of heating may be used, for example resistive heating (the Joule effect).
A gas containing one or more precursors of carbon, typically hydrocarbons such as methane and/or propane, is admitted into the enclosure 10. In the example shown, admission takes place through the bottom 10a of the enclosure. The gas passes through a preheater zone 18 formed by one or more pierced plates disposed one above another in the bottom portion of the enclosure, beneath the plate 11 supporting the stack of substrates. The gas heated by the preheater plates (which are raised to the temperature that exists inside the enclosure) flows freely into the enclosure, passing simultaneously into the inside volume 24, into the outer volume 26, and into the gaps 22.
The residual gas is extracted from the enclosure by suction through an outlet formed in the cover 10b. 
A drawback of such a disposition is that a relatively small fraction of the gas flows in the gaps 22, which means that the substrates 20 are poorly fed with reactive gas since their large faces are adjacent to the gaps 22.
In order to avoid that drawback, U.S. Pat. No. 5,904,957 proposes modifying the disposition of FIGS. 1 and 2 in the manner shown diagrammatically in FIGS. 3 and 4.
The gas admitted into the enclosure and leaving the preheater zone 18 is channeled by a wall 19 into the inside volume 24 of the stack of substrates 20, and the inside volume 24 is shut off by a wall 25 at its end opposite from the end where the gas is admitted. The outlet for residual gas from the chamber 10 communicates with the outer volume 26.
As a result, the flow of gas is directed so as to flow from the inside volume 24 towards the outer volume 26 by passing through the pores of the substrates 20 and also through the gaps 22, between the radial spacers 30.
With that “directed-flow” type method of chemical vapor infiltration, the substrates 20 are fed better with the reactive gas. The gaps 22 between the substrates leave passages for the gas such that the pressures in the inside volume 24 and in the outer volume 26 are equal.
A similar result could be obtained by causing the gas to flow in the opposite direction, i.e. from the outer volume 26 towards the inside volume 24, the outer volume 26 being closed at its end opposite from the end where the gas is admitted, and the inside volume 24 communicating with the outlet for removing residual gas from the enclosure.
Another disposition, as shown in FIGS. 5 and 6, is proposed in document EP 0 792 385.
That disposition differs from the disposition of FIGS. 3 and 4 in that the gaps 22 between the substrates are closed off by using annular spacers 32 disposed beside the inside diameter or, as shown, beside the outside diameter of the substrates 20.
The gas is thus forced to flow from the inside volume 24 to the outer volume 26 by passing through the pores of the substrates 20, and a pressure difference is established between said two volumes. The method of densification by chemical vapor infiltration implemented under such circumstances is said to be of the “pressure gradient” type.
Compared with the equal pressure method of infiltration as implemented in the disposition of FIGS. 1 and 2, the method with a pressure gradient and forced gas flow enables densification to be performed more quickly.
However, the process is difficult to implement. As specified in document EP-0 792 385, the substrate 20 must be loaded into the enclosure 10 with great care in order to avoid gas leaking from the bottom of the stack, at the outlet from the preheater zone, between adjacent substrates, and also from the top of the stack. The wall 25 may be surmounted by a weight 25a serving to press it down in leaktight manner on the top of the stack by opposing the higher pressure that exists inside the stack.
In addition, when the process is continued beyond a threshold of substrate densification, leading to the pressure in the center of the stack becoming too high, the microstructure of the matrix has been observed by the applicant to become modified, and indeed large quantities of soot can be formed. Those phenomena are undesirable since they lead to a change in the properties of the material which can be harmful while it is in use. Furthermore, they can require the densification process to be stopped before the desired density level has been reached. It is then necessary subsequently to finish off densification, for example in a final chemical vapor infiltration step performed under equal pressures, as stated in document EP-0 792 384. In addition, the increase of pressure inside the stack as densification progresses causes the stack to swell, and that can have destructive effects.